In the years since the appearance of Razdan's ground-breaking review on the synthesis of cannabinoids, research activity has continued apace (Razdan, in ApSimon, ed., The Total Synthesis of Natural Products, Vol. 4, pp. 185-262, New York, N.Y.: Wiley and Sons (1981); Huffman et al., Current Med. Chem., 3:101-116 (1996)). The interest which this area engenders is due in part to the challenge which the structures pose to the synthetic organic chemist, and also because of the diverse and useful pharmacological activities which many of these materials express. The chemical structures of the naturally occurring tricyclic cannabinoids, typified by delta-9-tetrahydrocannabinol (delta-9-THC), are very simple: there are only two stereogenic carbon atoms, two carbocycles and the dihydrobenzopyran ring. The functionality is in most cases limited to the phenolic Cl hydroxyl and to one or two oxygen-bearing functional groups. One would be justified in questioning whether this class of compounds is of sufficient complexity to continue to interest the organic chemist. The difficulties of the synthesis belie the simplicity of the structure, and are due, at least in part, to the following: (a) the materials are typically non-crystalline, and are often quite difficult or impossible to separate and purify without recourse to HPLC; (b) the aromatic portion of the molecule is very sensitive to oxidation, particularly in the presence of base or transition metals (see Hodjat-Kashani et al., Heterocycles, 24:1973-1976 (1986)); and (c) the delta-9-unsaturation is thermodynamically disfavored relative to delta-8-unsaturation. There is also no general method by which to favor delta-9-unsaturation kinetically.
Interest in the pharmacology of these materials goes back many thousands of years (Abel, Marijuana: The First Twelve Thousand Years, pp. 11-12, New York and London: Plenum Press (1980)). Herodotus' account of the Scythians' use of Cannabis sativa as an intoxicant makes it clear that the psychotropic properties of the producing plant were recognized since antiquity (Herodotus, The Histories, Book IV, pp. 295, Penguin Books, Ltd., Middlesex (1972)). In addition to uses as anaesthetics, spasmolytics, and hypnotics, cannabinoids have been used to combat emesis and nausea induced by cancer chemotherapy, and also in the treatment of glaucoma. In recent times, cannabinoids have achieved a certain notoriety due to their abuse potential. A significant portion of the synthetic effort has been directed toward the preparation of some of the oxygenated human urinary metabolites of delta-9-THC for use in forensic science as analytical standards for the detection of marijuana use.
Several developments have contributed to the current resurgence in interest in this area. The identification of the first cannabinoid receptor (CB1) in rat brain (Devane et al., Mol. Pharmacol., 34:605-613 (1988)) was a major advance. The identification of a second, peripheral, receptor subtype in splenocytes (CB2) (Munro et al., Nature, 365:61-65 (1993)), as well as the discovery of arachidonylethanolamine (anandamide) as the endogenous ligand for CB1 (Devane et al., Science, 258:1946-1949 (1992)), has made the story much more interesting. Involvement of the pharmaceutical industry has resulted in synthesis and evaluation of large numbers of analogs, and in the discovery of the first receptor antagonist.
The present invention is directed to overcoming the above-noted deficiencies in the art.